how to Create harmony patch?

[Mike Martone]

Hey guys, I am a piano player and I have a Yamaha DGX-630 (or something like that, its not here, but I think thats what it is) and a MacBook Pro. I need to take the regular piano sound and remap the keys so, for instance, there are two octaves but instead of each playing a different octave it is the same octave. The goal being to simulate tremolo picking on a guitar but playing an octave tremolo on the keyboard.

and also how do program a patch and what software to use (Mainstage 2?) to program it so you can hit one key and you now play a 5th or sim.

thanks so much